Magnetization prepared magnetic resonance imaging is a commonly used methodology whereby a combination of radio frequency (RF) pulses, gradient pulses, and temporal delays are used to prepare the longitudinal magnetization (Mz) of a spin system to a target state. The prepared longitudinal magnetization is converted to an image, in which the signal intensities are related to the starting Mz, using an imaging readout that comprises a train of RF pulses with constant flip angles. In quantitative magnetization prepared imaging, multiple magnetization prepared images are usually acquired using a systematically varied preparation scheme to result in prepared Mz that is a known function of the physical parameter of interest. The signal intensities from the resulting images can be fit to this function to determine the parameter of interest.
An underlying assumption made in quantitative magnetization prepared imaging is that the image signal intensity is directly proportional to the longitudinal magnetization prior to the imaging readout. Typical imaging comprises a series of RF pulses of constant flip angle, with single lines of k-space acquired sequentially between RF pulses. Image signal intensity is determined by the transverse magnetization when the center line of k-space is acquired, usually after numerous RF pulses when k-space is acquired linearly starting from the outer edge of k-space. While the transverse magnetization after the first imaging RF pulse is proportional to the starting Mz, subsequent acquisitions are not directly proportional due to cumulative effects of multiple preceding RF pulses and increased time for T1 relaxation. The magnitude of error in this relationship is affected by the flip angles and excitation phases of all preceding readout RF pulses, the patterns of gradients, as well as the relaxation properties of the spin system.
Imaging readouts may begin with a small number of “catalyzation” or “dummy” RF pulses that may have different flip angles from the rest of the imaging RF pulses and for which k-space data is not acquired. The primary purpose of these catalyzation pulses is to reduce oscillations in transverse magnetization during the subsequent constant flip angle train where k-space is acquired to reduce image artifacts.
Variable flip angle (VFA) readouts during the imaging RF train itself have been proposed to maximize the signal yield in hyperpolarized imaging experiments, to maintain constant transverse magnetization during imaging to reduce image blurring, or to optimize contrast between tissues with different relaxivities. In these approaches, a magnetization time-course is specified and a VFA scheme is designed to achieve it. For quantitative magnetization prepared imaging, VFA schemes should instead be formulated to improve the proportional relationship between the starting magnetization and the image signal intensity for a wide range of starting Mz and relaxation values.
Reduced flip angles can be used to lessen the effect of the RF pulses on the system's magnetization at the expense of reduced signal to noise or the desired image contrast. Therefore while a reduced, but still constant, flip angle readout may result in more accurate quantitative measurements, the loss of precision or image contrast may not be an acceptable tradeoff.